fix: TestCreateCollectorProxy unit test failing on go-tip

[Saumya40-codes]

Which problem is this PR solving?

• Resolves Unit tests fail on go-tip #6198

Description of the changes

• After the addition of t.Parallel in cmd/agent/app/builder_test.go in TestCreateCollectorProxy, its test started to fail during random ittterations. This was because in /cmd/agent/app/reporter/connect_metrics.go under NewConnectMetrics function we are doing expvar.NewString("gRPCTarget") which isnt thread-safe and it should not be changed on initialized
• Now we use sync.Once to tackle the above bug/problem

How was this change tested?

• running go test -v

Checklist

• I have read https://github.com/jaegertracing/jaeger/blob/master/CONTRIBUTING_GUIDELINES.md
• I have signed all commits
• I have added unit tests for the new functionality
• I have run lint and test steps successfully
  • for jaeger: make lint test
  • for jaeger-ui: yarn lint and yarn test

[yurishkuro]

Thanks!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.47%. Comparing base (d43fc84) to head (010164f).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6204      +/-   ##
==========================================
- Coverage   96.49%   96.47%   -0.02%     
==========================================
  Files         354      354              
  Lines       20127    20127              
==========================================
- Hits        19421    19418       -3     
- Misses        522      524       +2     
- Partials      184      185       +1     

Flag	Coverage Δ
badger_v1	8.31% <ø> (ø)
badger_v2	1.67% <ø> (ø)
cassandra-4.x-v1	14.39% <ø> (ø)
cassandra-4.x-v2	1.61% <ø> (ø)
cassandra-5.x-v1	14.39% <ø> (ø)
cassandra-5.x-v2	1.61% <ø> (ø)
elasticsearch-6.x-v1	18.59% <ø> (ø)
elasticsearch-7.x-v1	18.68% <ø> (ø)
elasticsearch-8.x-v1	18.85% <ø> (ø)
elasticsearch-8.x-v2	1.67% <ø> (+0.01%)	⬆️
grpc_v1	9.47% <ø> (-0.01%)	⬇️
grpc_v2	6.99% <ø> (-0.01%)	⬇️
kafka-v1	8.88% <ø> (ø)
kafka-v2	1.67% <ø> (ø)
memory_v2	1.67% <ø> (ø)
opensearch-1.x-v1	18.73% <ø> (ø)
opensearch-2.x-v1	18.73% <ø> (ø)
opensearch-2.x-v2	1.67% <ø> (ø)
tailsampling-processor	0.46% <ø> (ø)
unittests	95.38% <100.00%> (-0.02%)	⬇️

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